nRF52 WLCSP PCB Cost Very High: How to Reduce?

[Bill Finger]

We have recently completed the design for a product prototype using the 0.4 mm pitch nRF52832 WLCSP. When we sent it to the board production house, the cost to produce it was exorbitant, particularly when compared with our previous prototype which used the QFN part.

It seems that there are challenges using typical low cost PCB assembly techniques with 0.4mm pitch devices. Yet it would seem that this product is intended for low cost applications. How can we use this form factor and still produce inexpensive devices?

[Bwalker]

This is an old question, but just for anyone interested, a big part of the cost will be using blind vias. If you use non-blind vias, it will mean that you cant use pins other than the outer ones, but nordic designed the package so that all the important pins (pwr, gnd, antenna etc) are accessible in this manner. Unless you need lots of IO, it may be worth considering this.

[AmbystomaLabs]

Traditionally the best way to handle fine pitch BGA styled parts such as WLCSP would be with HDI (ie, laser drilled) vias. On HDI designs only the outer two layers are laser drilled and the entire board also has traditional through board drilled vias. The beauty of the laser drilled via is that since they are so small and have no remaining hole that would wick solder. Due to this you can place them directly in the center of each pad on a WLCSP. It makes the RF a little more challenging since the outer layer dielectric might be only 3 mils so a 50 ohm strip can be pretty narrow.

Still not a cheap board to make. But a very elegant solution and the price of HDI boards keeps coming down.